Insights into material extrusion in additive manufacturing: Advancing the understanding of mechanical properties and interlaminar behaviour of fibre-reinforced thermoplastics

Material extrusion is one of the most widely used three-dimensional printing
techniques, owing its success to its simplicity and relatively low cost. The
technique allows the manufacturing of polymer parts with complex shapes in a
layer-by-layer fashion according to a computer-aided design, without the use of
expensive moulds and while minimising material waste. The technique is
traditionally used for rapid prototyping of on-demand products and other
applications for which low to medium volumes are manufactured. However, due
to the possibility of creating intricate geometries, the application field of the
technique has broadened towards structural, load-bearing composite parts in the
automotive, aerospace and biomedical industries.
While the material extrusion printing technique allows the selection from a broad
range of materials and material properties, its multiple inherent weaknesses still
cast a shadow over the mechanical performance of the printed parts. This thesis
aims to address three of these challenges.
Firstly, the naturally weak mechanical properties of thermoplastic polymers do
not suffice for structural parts, and therefore thermoplastic parts have been mostly
used for non-structural and low-performance applications. This significant
limitation has driven researchers towards the reinforcement of thermoplastic
filaments with short, long or continuous fibres.
Secondly, printed parts suffer from inherently weak interlayers. This is due to a
combination of the fast cooling of the substrate during the printing process and
the calorimetric properties of the polymer. In addition, there is a lack of
thermomechanical consolidation during or after printing, as opposed to
conventional manufacturing methods for composites. As laminated composites
are already susceptible to delamination damage without the additional complexity
of the printing technique, assessment of the interlaminar properties of printed parts
is critical. Post-printing consolidation could remedy this issue by reducing the
void content in the laminate.
And thirdly, some commonly used materials in material extrusion such as
polyamides or aramid fibres are hygroscopic, which severely limits their
application in environments where temperature and moisture may vary. The
hygrothermal effect on the mechanical performance of parts printed with these
new filament materials needs to be understood, such that these materials can be
employed in a wide range of applications.
The detailed investigation in this PhD thesis presents new insights into the
mechanical performance and interlaminar behaviour of additively manufactured
fibre-reinforced thermoplastics, contributing to the understanding of the
characteristics of these emerging materials.